Handling a request to establish a packet switched session

ABSTRACT

The invention relates to a method for handling a request by a first communication device  11  connected to a network of a communication system to establish a packet switched session with a second, wireless communication device  12,  which session requires the second communication device  12  to be connected to a network which enables the establishment of the session. In order to enable an enhanced handling of such a request, it is proposed that it is first determined whether the second communication device  12  is currently connected to such a network. In case it is determined that the second device  12  is not connected to such a network, a corresponding indication is transmitted to the first device  11.  The user of the first device  11  is then informed via the first device  11  that the requested session cannot be established. The invention relates equally to a communication system enabling such a method and to corresponding elements  11, 13  of such a system.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority under 35 USC 119 toInternational Application PCT/IB02/01348, filed Apr. 16, 2002.

FIELD OF THE INVENTION

[0002] The invention relates to a method for handling a request by afirst communication device connected to a network of a communicationsystem to establish a packet switched session with a second, wirelesscommunication device, which session requires the second communicationdevice to be connected to a network of the communication system, whichnetwork enables the establishment of the requested session. Theinvention relates equally to a network element, to a communicationdevice and to a communication system comprising at least one networkelement and one communication device.

BACKGROUND OF THE INVENTION

[0003] 3G (3rd generation) communication systems will enable new kindsof calls with new features by providing packet switched (PS) connectionsvia PS networks. Such calls can be in particular calls in the course ofwhich multimedia content is transmitted, e.g. video calls, gamingsessions, etc.

[0004] According to 3GPP (3rd generation partnership project), whichoffers specifications for 3G systems, a Session Initiation Protocol(SIP) is to be employed for call control and signaling functions whenInternet Protocol (IP) technology is to be used end-to-end fordelivering multimedia content to mobile terminals.

[0005] SIP is an IETF (Internet Engineering Task Force) protocol whichis defined e.g. in the Request for Comments (rfc) 2543bis-09: “SIP:Session Initiation Protocol”, of Sep. 4, 2000, which is incorporated byreference herein.

[0006] SIP constitutes an application-layer control protocol that canestablish, modify and terminate multimedia sessions or calls between twoor more parties in an IP network. To this end, a variety of SIP messagesare defined. A SIP message can be either a request from a client to aserver, or a response from a server to a client. SIP allows for instanceto determine the willingness of a called party to engage in a requestedcommunication and to set up a call by establishing call parameters atboth, called and calling party.

[0007] However, also conventional circuit switched systems will furtherexist within the 3G communication systems. The 3G specific featuresrelying on a PS connection can obviously not be used with communicationdevices connected to a circuit switched (CS) network of thecommunication system.

[0008] A communication device may be connected to a circuit switchednetwork of a 3G communication system, because it is a CS device which isonly capable of accessing circuit switched networks. But also PS devicesmay roam from a PS network to a CS network, e.g. from UMTS (universalmobile telecommunication services) to GSM (global system for mobilecommunications), in particular as long as the PS networks do not coverall areas.

[0009] A calling party may therefore unintentionally try to establish a3G specific session with a called party which is at least currentlyconnected to a CS network. The calling party is not able to know towhich kind of network a called PS device is currently connected, andsometimes, a calling party may not be aware that the called party isusing a CS device.

[0010] When a session requiring a PS connection is requested to beestablish with such a device connected to a CS network, an establishmentof the connection will be tried, but fail and be dropped. This implies awaste of resources of the connecting networks and of the deviceconnected to a CS network. Further, the user of the requesting terminalis not aware of the reason for which the requested session is notestablished.

SUMMARY OF THE INVENTION

[0011] It is an object of the invention to enhance the handling of arequest by a first communication device connected to a network of acommunication system to establish a packet switched session with asecond, wireless communication device, which session requires the secondcommunication device to be connected to a network of the communicationsystem which enables the establishment of the requested session.

[0012] This object is reached according to the invention with a methodcomprising as a first step determining whether the second communicationdevice is currently connected to a network enabling the establishment ofthe requested session. In case the second communication device is atleast currently not connected to a network enabling the establishment ofthe requested session, a corresponding indication is transmitting to thefirst communication device in a second step. In a last step, a user ofthe first communication device is informed via the first communicationdevice that the requested session cannot be established.

[0013] The object of the invention is equally reached with a networkelement for a network of a communication system, which comprises meansfor receiving from a first communication device a request to establish apacket switched session with a second, wireless communication device,which session requires said second communication device to be connectedto a network of the communication system which enables the establishmentof the requested session. The proposed network element further comprisesmeans for determining whether the second communication device iscurrently connected to a network enabling the establishment of therequested session. Moreover, the network element of the inventioncomprises means for transmitting an indication to the requesting firstcommunication device that the second communication device is currentlynot connected to a network enabling the establishment of the requestedsession, if this was determined.

[0014] The object of the invention is also reached with a communicationsystem comprising at least such a network element of a communicationnetwork and at least one communication device. The communication deviceof the proposed communication system can access the communicationnetwork. It moreover includes means for requesting the establishment ofa packet switched session with another, wireless communication device,which session requires that the other communication device is connectedto a network of the communication system which enables the establishmentof the requested session. The communication device further comprisesmeans for receiving a response indicating that the other communicationdevice is currently not connected to a network enabling theestablishment of the requested session. The communication deviceincludes in addition means for informing a user that the requestedsession cannot be established.

[0015] The object of the invention is finally reached with acommunication device which is able to access a network of acommunication system. The communication device according to theinvention comprises means for requesting the establishment of a packetswitched session with another, wireless communication device, whichsession requires that the other communication device is connected to anetwork of the communication system which enables the establishment ofthe requested session. The device further comprises means for receivinga response to a request for a session with another communication device,which response indicates that the other communication device iscurrently not connected to a network enabling the establishment of therequested session. In addition, the communication device according tothe invention comprises means for detecting in a received response to arequest for a session with another communication device an indicationthat the other communication device is currently not connected to anetwork enabling the establishment of the requested session and fortriggering a function associated in the communication device to thedetected indication. The triggered function can consist in any desirablereaction to the indication that a requested session cannot beestablished, including a notification to the user of the communicationdevice.

[0016] The invention proceeds from the idea that a user initiating a PSsession via a communication device could be informed in case the sessioncannot be established, whenever the called party is roaming in networkswhich do not enable the establishment of the requested session. Thereason for this might be that the communication device of the calledparty does not support packet switched sessions at all or only in alimited way, and is thus not able to roam in the kind of networksrequired for the requested session. Further, a communication devicewhich principally supports the establishment of the requested sessionmight currently be connected to a network, like a CS network, which doesnot enable the establishment of the requested PS session. In order toenable the notification to the calling party, it is therefore determinedin the network to which kind of network the communication device of thecalled party is currently connected.

[0017] It is an advantage of the invention that it is more convenient tothe calling party, since the calling party is informed about theimpossibility of establishing the desired session. It is further anadvantage of the invention that it is simple and easy to develop.Further, it allows to save resources of the called communication deviceand of the network, since the establishment of the session can beinterrupted at an early point of time, i.e. before it fails.

[0018] In an advantageous embodiment of the invention, the informationthat the requested session cannot be established is presented to a useron a display of the first communication device.

[0019] The information can be presented for example as an icon, an alertand/or an advertisement. The data required for the presentation does nothave to be stored necessarily in the first communication device itself.Preferably, the information to the first communication device indicatingthat the second communication device is currently not connected to anetwork enabling the establishment of the requested session rathercomprises an identification of a location from which data for presentingthe information that the requested session cannot be established can bedownloaded. This data may belong in particular to an icon or an applet.It can be stored for instance at some server which the communicationdevice is able to access via the network to which it is connected.

[0020] In addition to the actual information that a requested sessioncannot be established, the user may be provided with different optionsfor a further proceeding. These options may be presented as well on thedisplay of the communication device. The offered options can include forexample canceling the request and/or establishing a voice connectioninstead of the requested connection before the network drops theinitiated session.

[0021] The information to the first communication device indicating thatthe second communication device is currently not connected to a networkenabling the establishment of the requested session is preferablyprovided by a network element of the network to which the firstcommunication device is connected.

[0022] The information may be included for example in a header of aresponse message transmitted to the first communication device.

[0023] In a further preferred embodiment of the invention, theestablishment of the session is based on the SIP using SIP messages. Inthis case, existing mechanisms can be made use of and it is notnecessary to add new features to existing protocols.

[0024] The request to establish a session can be included in particularin a SIP INVITE message. The INVITE message indicates according to theabove cited rfc 2543bis-09 that a user or a service is being invited toparticipate in a session. The message body of an INVITE message mayfurther contain a description of the session to which the called partyis being invited.

[0025] The information to the first communication device indicating thatthe second communication device is currently not connected to a networkenabling the establishment of the requested session may be included inparticular in a SIP Informational response message. Such a SIPInformational response message indicates according to the above citedrfc 2543bis-09 that a contacted server or proxy is performing somefurther action and does not yet have a definitive response. The employedSIP Informational response message can be for instance a SIP 100 TRYINGresponse message to the first communication device. A 100 Tryingresponse message indicates according to the above cited rfc 2543bis-09that some unspecified action is being taken on behalf of a call, but thecalled party has not yet been located.

[0026] The information to the first communication device indicating thatthe second communication device is currently not connected to a networkenabling the establishment of the requested session is furtherpreferably included in a header of a SIP response message, e.g. of theSIP 100 TRYING response message.

[0027] For SIP messages, several obligatory and optional headers aredefined in the above cited rfc 2543bis-09. It is proposed that for thepurposes of the invention, a CALL-INFO header and/or a WARNING header ofa SIP response message is employed.

[0028] The CALL-INFO general header field can be used according to theabove cited rfc 2543bis-09 for providing additional information aboutthe caller or the called party, depending on whether it is found in arequest or response. A CALL-INFO header may contain in particular an URI(uniform resource indicator) to supplementary call information on aserver. The purpose of the respective URI is described by a “purpose”parameter in the header. The “purpose” parameter “info” describes thecaller or called party in general. The CALL-INFO header thus provides aflexible possibility to provide a communication device with information.

[0029] For the invention, a URI inserted into a CALL-INFO header maypoint to a location from which data for an icon or an applet can bedownloaded, which icon or applet are to be used for informing the userof a device that a called party is not connected to a network enablingthe establishment of the requested session. When a communication devicereceives such a CALL-INFO header, it is then able to retrieve the datafor supplementary information from the server using an additionalsuitable protocol, e.g. HTTP (hypertext transport protocol), which is anIETF standard for distributed, collaborative, hypermedia informationsystems.

[0030] The CALL-INFO header may provide moreover some additional contentat the application level.

[0031] The WARNING header defined in the above cited rfc 2543bis-09 canbe employed in addition, in case a header is desired which enables atriggering of more extensive functions.

[0032] The WARNING header field is intended to carry additionalinformation about the status of a response. WARNING headers are sentwith responses and contain a three digit warning code, a host name, anda warning text. In case the first digit of the warning code is a “3”,the WARNING header comprises a warning specific to SIP. Warning codes300 through 329 are reserved for indicating problems with keywords inthe session description, warning codes 330 through 339 are reserved forwarnings related to basic network services requested in the sessiondescription, warning codes 370 through 379 are reserved for warningsrelated to quantitative QoS parameters requested in the sessiondescription, and warning codes 390 through 399 are reserved formiscellaneous warnings that do not fall into one of the abovecategories. Additional warning codes can be defined according to theabove cited rfc 2543bis-09 through IANA (Internet Assigned NumbersAuthority), thus also a new warning-code for indicating that a calleduser is roaming in a CS network. The warning text in the WARNING headershould be in a natural language that is most likely to be intelligibleto the human user receiving the response.

[0033] The information included in the WARNING header may triggercertain functions in the receiving communication device, which willdepend on the application that is currently running. Thus, someintelligence is added to the communication devices. The WARNING headeris used by the communication device without notification to the user. Itmight be useful to trigger some additional functionality based on thecode indicated in the header. The warning is interpreted as part ofcertain exceptional happenings in the type of session that is to beestablished. The warning code may for example inform an application thatthe called communication device is currently under CS domains using GSM,in which packet switched multimedia services are not supported. Thus, incase the application is trying to send Rich Call information such asClips, attachments etc. as part of the session set up, it will be awaredue to the information in the WARNING header that the othercommunication device will not be able to handle Rich Call information.The application will thus refrain from sending Rich Clip information,since this would result either in a call attempt failure or the contentwill dropped at some point by the network. Similarly, the warning codemay inform an application that the called communication device isroaming in a kind of GPRS network which supports PS connections, butwhich might not have SIP capabilities or which might not support packetswitched multimedia services. Also in this case, the application willknow that specific added value transmitted in the call set-up will belost.

[0034] In case the information to a user indicating that a requestedsession cannot be established is included in an optional header, likethe SIP CALL-INFO header and the WARNING header, interoperabilityproblems are avoided. Any communication device that is not capable ofinterpreting the header can simply ignore it. Thus, only communicationdevices that are able to understand the feature will provide a servicefacilitating the announcement that a called party is not connected to anetwork enabling the establishment of the requested session.

[0035] In the first communication device, advantageously a user agent(UA) takes care of presenting the information that the device of acalled party is not connected to a network enabling the establishment ofthe requested session. A user agent is an application which can act bothas a user agent client and user agent server. The user agent can alsoaccess a data source, for example an indicated HTTP server, to retrievedata required for presenting the information.

[0036] A network element taking care of informing the communicationdevice that a requested session cannot be established may not knowitself whether or not the communication device of the called party isnot connected to a network enabling the establishment of the requestedsession. Therefore, such a network element preferably comprises meansfor inquiring from a further network element of the communication systemto which kind of network the other communication device is currentlyconnected. The first network element comprises in this case moreovermeans for receiving a response to the query, and means for evaluatingthe response to the query. The further network element can be forexample a network element of a public switched telephone network (PSTN),while the first network element may be a network element of a UMTSnetwork. The invention can thus relate also to UMTS-PSTN interworking.

BRIEF DESCRIPTION OF THE FIGURES

[0037] Other objects and features of the present invention will becomeapparent from the following description of an exemplary embodiment ofthe invention considered in conjunction with the accompanying drawings,of which

[0038]FIG. 1 illustrates an embodiment of the method according to theinvention implemented in a communication system according to theinvention;

[0039]FIG. 2 shows an exemplary presentation of information according tothe invention on a display of a mobile phone; and

[0040]FIG. 3 shows a network element such as the SIP proxy of FIG. 1,which may be used to carry out the methodology, according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0041]FIG. 1 presents selected elements of a communication system inwhich the present invention can be realized.

[0042] The communication system comprises a first phone 11 used by afirst user and a second phone 12 used by a second user. The first phone11 is a SIP phone which is currently connected to a PS network, e.g. aUMTS network, while the second phone 12 is a fixed CS phone connected toa CS network, i.e. PSTN. The second phone 12 could also be a SIP phonewhich is only currently connected to a CS network, e.g. a mobile SIPphone currently roaming in a GSM network or another kind of CS mobilephone such as a GSM phone.

[0043] The PS network comprises a SIP proxy 13 and a Home SubscriberServer (HSS) 14 which is accessible by the SIP proxy 13. A HSS is aserver in a 3G network which stores information for the terminals, suchas security credentials, service profiles and routing area informationused for locating a terminal. The HSS 14 thus comprises information onthe current location of all phones 11, 12 of the communication system.The SIP phone 11 is able to communicate with the CS phone 12 via the SIPproxy 13 and connecting networks 15 in voice sessions, indicated in thefigure with an arrow with a dashed line, but not in sessions requiring aPS connection, like gaming etc.

[0044] The first user assumes erroneously that the phone 12 employed bythe second user is also a SIP phone roaming in PS networks and desiresto establish a session which requires a PS connection with the user ofthe CS phone 12.

[0045] The first user therefore initiates the desired session bytransmitting a SIP INVITE message including the phone number of thesecond user via the SIP phone 11. The transmission of the INVITE messageis indicated in FIG. 1 as a first step 1). Alternatively, the first usermight transmit a SIP URL including the phone number of the second userphone 11, e.g. “sip:+358401234568@sonera.fi”, for initiating the desiredsession.

[0046] The SIP Proxy 13 receives this request from the SIP phone 11.Thereupon, the SIP proxy 13 transmits a query to the HSS 14 inquiringabout the current location of the phone 12 identified in the INVITEmessage by a phone number of its user. The transmission of the query tothe HSS 14 is indicated in FIG. 1 as a second step 2).

[0047] The HSS 14 determines based on the received information thecurrent location of phone 12. In a third indicated step 3), the HSS 14informs the SIP proxy 13 that the second user is roaming at leastcurrently in a CS network.

[0048] As a result, the SIP proxy 13 assembles a 100 TRYINGinformational SIP response and sends it to the SIP phone 11 of the firstuser, before trying to establish the session with the CS phone 12 viathe connecting networks 15. The transmission of the 100 TRYING isindicated in FIG. 1 as a fourth step 4).

[0049] The transmitted 100 TRYING contains a CALL-INFO header and aWARNING header. The CALL-INFO header has for example the format“<http://www.nokia.com/cs.app>; purpose=info”. The first part of theCALL-INFO header constitutes a URI to an icon or an applet on a specificHTTP server. The “purpose” parameter “info” describes the called partyin general. The WARNING header has for example the format “3XX nokia.com‘CS roaming’”, wherein “3XX” stands for a three digit number warningcode that was defined through the IANA specifically for indicating thata called user is roaming in a CS network.

[0050] “nokia.com” stands for the host name, and “‘CS roaming’” is therequired warning text of the WARNING header.

[0051] The SIP phone 11 receives the 100 TRYING and the associatedCALL-INFO header and WARNING header.

[0052] A user agent of the SIP phone 11 knows from the information inthe CALL-INFO header that the phone 12 of the called party is at leastcurrently connected to a CS network and retrieves the icon or applet ofwhich a source is identified by the URI in the CALL-INFO header from thecorresponding HTTP server. The HTTP server is not shown in FIG. 2.Further, the SIP phone 11 displays the fetched icon on its display orruns the fetched applet.

[0053] The icon or the applet presents an information to the user of theSIP phone 11 indicating that the called phone 12 is at least currentlyconnected to a CS network. Thus, the first user is informed that thesecond user is at least currently not capable of participating in morethan a voice session.

[0054] The SIP phone 11 further presents on its display options enablinga user to cancel the requested session or to establish e.g. a voicesession instead of the requested session. In case the first user wasinterested only in a session requiring a PS connection, he/she may nowselect the first option and cancel the session, in order to avoidwasting resources in the network. In case the first user desires toestablish a voice session instead of the originally requested session,he/she can select the second option and keep on the phone 11 toestablish a ‘normal’ voice call with the second user via the CS phone12.

[0055] The information in the CALL-INFO header can be used by the SIPphone 11 in addition for some functionality at the application layer, inorder to provide the user with further information.

[0056] The WARNING header in the 100 TRYING message provides additionalinformation to the SIP phone 11 indicating that a session different fromthe requested session is being established. The WARNING header may beused by the phone 11 exclusively for terminal functionality. Anintelligent terminal can use the warning code “3XX” included in theWARNING header to trigger certain functions without notifying the user.

[0057]FIG. 2 shows an exemplary presentation on the display 21 of theSIP phone 11 of FIG. 1 after the 100 TRYING message was received.

[0058] The display 21 informs the user that a session is tried to beestablished by an indication “Calling . . . ” 22 and to which calledparty by an indication “User B” 23. Further, an icon 24 representing aCS phone, which is circled in the figure by a dashed line, informs theuser of the SIP phone that the called party “User B” uses a CS phone oris at least currently connected to a CS network. The data for this iconwas retrieved by the mobile phone as described above from a server. Itshould be understood that this particular icon constitutes only anexample.

[0059] Moreover, any other kind of alert could be used to inform theuser of the SIP phone that the called party is at least currentlyconnected to a CS network, e.g. a plain text or a JAVA applet etc. Thedisplay 21 further offers the user to view possible options forproceeding with the call by selecting a presentation “Options” 25 or toend the call by selecting a presentation “End” 26. When selecting thepresentation “Options” 25, one option might be presented for example forestablishing a normal voice call instead of the requested session. Thedisplay 21 may show in addition other elements which are not depicted inFIG. 2.

[0060]FIG. 3 shows a network element such as the SIP proxy 13 of FIG. 1which may be used to carry out the methodology described above. Thenetwork element 13 of FIG. 3 includes means 30 for receiving from afirst communication device 11 a request signal on a line 32 to establisha packet switched session with a second, wireless communication device12, which session requires the second communication device to beconnected to a network of the communication system shown in FIG. 1,which network enables the establishment of the requested session. Themeans 30 of FIG. 3 may provide a received signal on a line 30 a means 31for determining whether the second communication device 12 is currentlyconnected to a network enabling the establishment of the requestedsession. Although the means for receiving and the means for determiningare shown as separate functional blocks in FIG. 3, they could beimplemented in a single device or process. It should also be realizedthat if implemented as separate elements they may be coupled togethereither directly or through intervening devices. The means 31 fordetermining if the second device is connected to a network enabling theestablishment of the requested session may be used to determine if thesecond device is connected to a packet switched network. It can do thiseither by contacting the HSS 14 of FIG. 1 as shown by signal lines 2 aand 3 a in FIG. 3 or by inquiring from yet a further network element ofthe communication system to which kind of network the secondcommunication device is currently connected. FIG. 3 shows the internalsof such a device 32 for making inquiry 2 a by a means 32 for makinginquiry and means 33 b for receiving a response. Another device 34 has asimilar structure with devices 36, 38. In other words, the means 34 hasmeans 36 for inquiring from the further network element the type ofnetwork the second communication device is attached to. It also hasmeans 38 for receiving a response 3 b therefrom and for providing anindication thereof on a signal line 40 a to the means 31 for determiningif the second device is connected to a PS network or 40 b to a means 42for transmitting directly an indication to the first device that thesecond device is not in the packet switched network by a signal on aline 44.

[0061] Another signal on the line 44 may also be used earlier in thescenario to indicate to the first communication device that the proxy 13is “trying” to establish a connection. If the identification of thesecond communication device is not found in the HSS 14 of FIG. 1, thenthe device 34 of FIG. 3 may be used to make a similar inquiry to someintermediate network element connected between the proxy 13 of FIG. 1and the second communication unit 12 and residing within the network 15.

[0062]FIG. 3 also shows functional details of the SIP phone 11 in FIG. 1in more detail. It includes means 16 for requesting a packet switched(PS) session and providing the INVITE signal on the line 32 to the SIPproxy 13. It also includes means 17 for receiving the signal on the line44 from the SIP proxy 13 indicating that the proxy is “trying” or hasmade a determination that the second device 12 is not connected to a PSnetwork. The means 17 for receiving this response provides an outputsignal on a line 17 a to a means 19 for detecting a nonpacket switchedconnection in the second device. In turn, the means 19 provides a signalto means 21 for informing the user about this fact. This may be done byway of the display of FIG. 2. On the other hand, instead of directlysignaling the means 21, the means 19 for detecting the non-PS connectionmay instead send a signal on a line 19 a to a means 20 a for triggeringa function. This could be a variety of functions including thosementioned above. One of these functions is for example to retrieve datafrom outside the SIP phone 11 through some other network. Therefore, themeans for triggering the function 20 a sends a signal on a line 20 b toa means 20 c for retrieving data. The retrieval of such data is shown bya signal on a line 20 d that is routed through the SIP proxy 13 to thenetwork 15. Once the retrieved data is received back at the means 20 c,it provides the data on a line 20 e to the means 21 for informing theuser. This could be information retrieved from a URL or execution of anapplet obtained by this methodology.

[0063] It is to be noted that the described embodiment constitutes onlyone of a variety of possible embodiments of the invention. Thus,although the invention has been shown and described with respect to abest mode embodiment thereof, it should be understood by those skilledin the art that the foregoing and various other changes, omissions andadditions in the form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention.

1. Method for handling a request by a first communication device (11)connected to a network of a communication system to establish a packetswitched session with a second, wireless communication device (12),which session requires said second communication device (12) to beconnected to a network of said communication system which enables theestablishment of said requested session, said method comprising thefollowing steps: determining whether said second communication device(12) is currently connected to a network enabling the establishment ofsaid requested session; in case it is determined that said secondcommunication device (12) is at least currently not connected to anetwork enabling the establishment of said requested session,transmitting a corresponding indication to said first communicationdevice (11); and informing a user of said first communication device(11) via said first communication device (11) that said requestedsession cannot be established.
 2. Method according to claim 1, wherein auser of said first communication device (11) is informed that saidrequested session cannot be established by a presentation on a display(21) of said first communication device (11).
 3. Method according toclaim 1, wherein a user of said first communication device (11) isinformed by an icon (24), an alert and/or an advertisement that saidrequested session cannot be established.
 4. Method according to claim 1,wherein a user of said first communication device (11) is informed by anapplet running in said first communication device (11) that saidrequested session cannot be established.
 5. Method according to claim 1,wherein in addition to informing a user that said requested sessioncannot be established, a selectable option (25) is presented to saiduser via said first communication device (11) to establish a voice callinstead of said requested session and/or a selectable option (26) tocancel the request.
 6. Method according to claim 1, wherein saidindication transmitted to said first communication device (11) that saidsecond communication device (12) is currently not connected to a networkenabling the establishment of said requested session comprises anidentification of a location from which data for informing a user thatsaid requested session cannot be established can be downloaded by saidfirst communication device (11).
 7. Method according to claim 1, whereinsaid indication transmitted to said first communication device (11) thatsaid second communication device (12) is currently not connected to anetwork enabling the establishment of said requested session is includedin a header of a response message to said request.
 8. Method accordingto claim 1, wherein said request and said indication transmitted to saidfirst communication device (11) that said second communication device(12) is currently not connected to a network enabling the establishmentof said requested session comply with the Session Initiation Protocol(SIP).
 9. Method according to claim 8, wherein said request is a SIPINVITE message or a SIP URL and that said indication transmitted to saidfirst communication device (11) that said second communication device(12) is currently not connected to a network enabling the establishmentof said requested session is included in a header of a SIP responsemessage.
 10. Method according to claim 9, wherein said indicationtransmitted to said first communication device (11) that said secondcommunication device (12) is currently not connected to a networkenabling the establishment of said requested session is included in aSIP 100 TRYING response message to said first communication device (11).11. Method according to claim 10, wherein said indication transmitted tosaid first communication device (11) that said second communicationdevice (12) is currently not connected to a network enabling theestablishment of said requested session is included in a CALL-INFOheader and/or a WARNING header of a SIP message to said firstcommunication device (11).
 12. Network element (13) for a network of acommunication system, comprising means for receiving (30) from a firstcommunication device (11) a request to establish a packet switchedsession with a second, wireless communication device (12), which sessionrequires said second communication device (12) to be connected to anetwork of said communication system, which network enables theestablishment of said requested session; means for determining (31)whether said second communication device (12) is currently connected toa network enabling the establishment of said requested session; andmeans for transmitting (42) an indication to said first communicationdevice (11) that said second communication device (12) is currently notconnected to a network enabling the establishment of said requestedsession, in case said second communication device is currently notconnected to a network enabling the establishment of said requestedsession.
 13. Network element (13) according to claim 12, furthercomprising means for inquiring (33 a) from a further network element(14) of said communication system to which kind of network said secondcommunication device (12) is currently connected, means for receiving aresponse (33 b) to said query, and wherein said means for determining(31) whether said second communication device (12) is currentlyconnected to a network enabling the establishment of said requestedsession includes means for evaluating (30 a) said response to saidquery.
 14. Network element (13) according to claim 12, wherein saidmeans for transmitting an indication to said first communication device(11) that said second communication device (12) is currently notconnected to a network enabling the establishment of said requestedsession transmit an indication which comprises an identification of alocation from which data for informing a user that said requestedsession cannot be established can be downloaded by said firstcommunication device (11), in case said second communication device iscurrently not connected to a network enabling the establishment of saidrequested session.
 15. Network element (13) according to claim 12,wherein said means for transmitting an indication to said firstcommunication device (11) that said second communication device (12) iscurrently not connected to a network enabling the establishment of saidrequested session insert said indication into a header of a responsemessage to a request by said first communication device (11). 16.Network element (13) according to claim 12, wherein said means forreceiving from a first communication device (11) a request to establisha packet switched session with a second, wireless communication device(12) are suited to receive requests complying with the SessionInitiation Protocol (SIP), and wherein said means for transmitting anindication to said first communication device (11) that said secondcommunication device (12) is currently not connected to a networkenabling the establishment of said requested session transmit anindication complying with said SIP, in case said second communicationdevice is currently not connected to a network enabling theestablishment of said requested session.
 17. Network element (13)according to claim 16, wherein said means for transmitting an indicationto said first communication device (11) that said second communicationdevice (12) is currently not connected to a network enabling theestablishment of said requested session insert said indication into aheader of a SIP response message to a SIP INVITE message received bysaid means for receiving from a first communication device (11) arequest to establish a session with a second communication device (12).18. Network element (13) according to claim 17, wherein said means fortransmitting an indication to said first communication device (11) thatsaid second communication device (12) is currently not connected to anetwork enabling the establishment of said requested session insert saidindication into a SIP 100 TRYING response message to said firstcommunication device (11).
 19. Network element (13) according to claim18, wherein said means for transmitting an indication to said firstcommunication device (11) that said second communication device (12) iscurrently not connected to a network enabling the establishment of saidrequested session insert said indication into a CALL-INFO header and/ora WARNING header of a SIP message to said first communication device(11).
 20. Communication system comprising at least one network element(13) of a communication network according to claim 12 and at least onecommunication device (11) which is able to access said communicationnetwork and which includes means for requesting (16) the establishmentof a packet switched session with another, wireless communication device(12), which session requires said other communication device (12) to beconnected to a network of said communication system which enables theestablishment of said requested session, means for receiving (17) aresponse to such a request indicating that said other communicationdevice (12) is currently not connected to a network enabling theestablishment of said requested session, and means for informing (21) auser that a requested session cannot be established.
 21. Communicationsystem according to claim 20, wherein said communication device (11)further comprises means for detecting (19) in such a received responsean indication that said other communication device (12) is currently notconnected to a network enabling the establishment of said requestedsession and for triggering (20 a) a function associated in saidcommunication device (11) to said indication.
 22. Communication systemaccording to claim 21, wherein said communication device (11) furthercomprises means for retrieving data (20 b) from a location indicated ina received response, which data is used by said means (21) of saidcommunication device (11) for informing a user that a requested sessioncannot be established for indicating that the requested session cannotbe established.
 23. Communication system according to claim 20, whereinsaid means of said communication device (11) for informing a user that arequested session cannot be established comprise a display on which acorresponding information is presented.
 24. Communication systemaccording to claim 20, wherein said means of said communication device(11) for informing a user that a requested session cannot be establishedinform a user with an icon (24), an alert and/or an advertisement thatsaid requested session cannot be established.
 25. Communication systemaccording to claim 20, wherein said means of said communication device(11) for informing a user that a requested session cannot be establishedrun an applet for informing a user that said requested session cannot beestablished.
 26. Communication system according to claim 20, whereinsaid means of said communication device (11) for informing a user thatthe requested session cannot be established further present a selectableoption (25) to a user to establish a voice call instead of saidrequested session and/or a selectable option (26) to cancel the request.27. Communication system according to claim 20, wherein saidcommunication device (11) is capable of handling Session InitiationProtocol (SIP) messages.
 28. Communication device (11) which is able toaccess a network of a communication system, said communication device(11) comprising means for requesting (16) the establishment of a packetswitched session with another, wireless communication device (12), whichsession requires that said other communication device (12) is connectedto a network of said communication system which enables theestablishment of said requested session; means for receiving (17) aresponse to a request for a session with another communication device(12), which response indicates that the other communication device (12)is currently not connected to a network enabling the establishment ofsaid requested session; and means for detecting (19) in a receivedresponse to a request for a session with another communication device(12) an indication that the other communication device (12) is currentlynot connected to a network enabling the establishment of said requestedsession and for triggering (20 a) a function associated in saidcommunication device (11) to said indication.
 29. Communication device(11) according to claim 28, further comprising means for informing (21)a user of said communication device (11) that a requested session cannotbe established.
 30. Communication device (11) according to claim 29,further comprising means for retrieving (20 c) data from a locationindicated in a received response, which data is used by said means forinforming a user that a requested session cannot be established forindicating that the requested session cannot be established. 31.Communication device (11) according to claim 29, wherein said means forinforming (21) a user that the requested session cannot be establishedcomprise a display on which a corresponding information is presented.32. Communication device (11) according to claim 29, wherein said meansfor informing a user that a requested session cannot be establishedinform a user with an icon (24), an alert and/or an advertisement thatsaid requested session cannot be established.
 33. Communication device(11) according to claim 29, wherein said means for informing a user thata requested session cannot be established run an applet for informing auser that said requested session cannot be established. 34.Communication device (11) according to claim 29, wherein said means forinforming a user that a requested session cannot be established furtherpresent a selectable option (25) to a user to establish a voice callinstead of said requested session and/or a selectable option (26) tocancel said requested session.
 35. Communication device (11) accordingto claim 28, wherein it is capable of handling Session InitiationProtocol (SIP) messages.